1. Field of the Invention
This invention relates generally to frequency multiplexing/demultiplexing through digital processing.
2. Description of the Prior Art
The first frequency multiplexing/demultiplexing devices through digital processing were developed within transmultiplexers to solve the problem of data transfers between multiplex-ways of different types within the telephone network. For example, such a transmultiplexer sets up a connection between a secondary group (SG) multiplex-way consisting of 60 frequency multiplexed analog telephone channels, with 4 KHz bandwidth each one, and two primary MIC multiplex-ways (TN1) at 2,048 Mbits/sec. Thus, very schematically, the transmultiplexers provide a conversion between frequency division multiplex-way (FDM) and time division multiplex-way (TDM).
In the field of satellite telecommunications, this transmultiplexing technique has proved to be especially advantageous in meeting two conditions in view of optimum implementation of satellite networks. Firstly, the frequency division multiple access (FDMA) for up-links set up from earth stations to the satellite has proved to be a promising technique in that it reduces the transmission power needed and the dimensions of the transmitter antennas for the earth stations, thus offering greater flexibility for the station installation sites. Secondly, concerning the down-links set up from the satellite to the earth stations, the FDMA technique is recommended so as not to generate intermodulation noise when several carriers modulated by the signals to be transmitted are transmitted by satellite. This intermodulation noise increases depending on the number of carriers. A single carrier modulated by FDMA digital frame inhibits this intermodulation problem.
The satellites employed to make such a conversion between FDMA and TDMA techniques are commonly known as regenerative satellites given that they "regenerate" the component signals received in the form of a multiplex channel as a function of the individual frequency bands of the component signals.
As compared to a network of satellite microstations (VSAT for "Very Small Aperture Terminal") in which the respective links between the microstations and a central station and the central station and the microstations are provided by a TDMA carrier and a TDM carrier, the transmultiplexing technique on board regenerative satellites reduces by half the transmission time between two stations given that the data transfers between stations are made directly and not via a central station.
Despite all the aforesaid advantages resulting from the use of regenerative satellites, the satellite operators hesitate somewhat to use them.
This results in two requirements which must be satisfied in view of optimum use of the total bandwidth allocated to the network and which are antagonistic according to the prior art:
the transmultiplexer must be programmable "simply" from the ground via a dedicated channel, so as to be configured as a function of the evolution of the traffic in the satellite network, and
the transmultiplexer must receive frequency multiplexed signals with different bandwidths.
The central unit of the transmultiplexer is a frequency demultiplexing device which is programmed from the ground. According to the prior art, different demultiplexer structures, on a digital filter base, have been proposed. The following can be chiefly singled out:
the embodiments carrying out individual processings of the channels, which offer the drawback of needing highly complex and costly material embodiments when the number of frequency multiplexed carriers increases and which, moreover, necessitate unwieldy programming;
the "fast Fourier transform" embodiments which offer the drawback of only being really efficient when there is a large number of carriers, given that if this is not the case, they need long processing times as compared to the number of carriers; and
the embodiments with polyphase networks consisting of delay lines and digital filters which, according to the prior art, prevent the processing of frequency multiplexed signals with different bandwidths.